The eGFR test determines the glomerular filtration rate of the kidneys, which indicates how well the kidneys are functioning, states MedlinePlus. A blood sample is needed to determine eGFR, which is calculated using a formula that includes factors such as creatinine level, age, ethnicity, gender, height and weight.
The glomerular filtration rate estimates the amount of blood that passes through the glomeruli, or tiny filters inside the kidneys, each minute, states MedlinePlus. Low eGFR indicates that the kidneys are not filtering the blood well. An eGFR of less than 60 for three months or more is a sign of kidney disease. An eGFR less than 15 indicates kidney failure and requires immediate medical care.